1. Field of the Invention
The present invention relates generally to a method for the recovery of essentially pure zinc oxide and specifically to a method for the recovery of essentially pure zinc oxide in a recycling operation from metal dust containing zinc compounds.
2. Prior Art
Zinc oxide typically is a coarse white or greyish powder which has a variety of uses including as an accelerator activator, as a pigment, as a dietary supplement and in the semiconductor field. Zinc oxide is found in commerical by-products including waste material streams such as fly ash and flue dust. Methods for recovering zinc oxides are known in the art, including recovering zinc oxide from industrial waste materials. Such previous methods have included leaching with mineral acid, caustic soda, ammonium hydroxide, and ammonium carbonate solutions. However, these methods have low yields of zinc oxide and typically do not recovery pure zinc oxide, the recovered zinc oxide being contaminated with other metal salts. Therefore, in order to obtain pure zinc oxide, subsequent roasting and evaporation processes were necessary.
Burrows, U.S. Pat. No. 3,849,121, assigned to a principal of the assignee of the present invention, discloses a method for the selective recovery of zinc oxide from industrial waste. The Burrows method comprises leaching the a waste material with an ammonium chloride solution at elevated temperatures, separating iron from solution, treating the solution with zinc metal and cooling the solution to precipitate zinc oxide. The Burrows patent discloses a method to take metal dust which is mainly a mixture of iron and zinc oxides and, in a series of steps, to separate out the iron oxides and waste metals. However, the material obtained in the last step is a mixture of a small amount of zinc oxide, hydrated zinc phases which can include hydrates of zinc oxide and zinc hydroxide, as well as other phases and a large amount of diamino zinc dichloride Zn(NH.sub.3).sub.2 Cl.sub.2 or other similar compounds containing zinc and chlorine ions. Currently, the Burrows method is not economically viable because of Environmental Protection Agency guidelines established subsequent of the issuance of the Burrows patent. Additionally, the Burrows method is not a continuous method and, therefore, is not economical as a continuous process.
Thus, there exists a need for a method which will recover zinc oxide from industrial waste which results in a product the majority of which is zinc oxide, and not mixtures of zinc oxide and other zinc phases. The method disclosed below relates to the preparation of pure zinc oxide. In addition, since zinc oxide is the desired product and diamino zinc dichloride is undesired, the method disclosed herein demonstrates how to increase the formation of zinc oxide and decrease the formation of diamino zinc dichloride.
Waste metal process dust typically has varying amounts of lead, cadmium and other metals contained in the dust. For various reasons, it is desirable to remove such metals from the waste metal dust, for example to recycle the lead and cadmium and/or to prevent introduction of the lead and cadmium into the atmosphere. The Burrows patent includes a method for removing dissolved lead and cadmium from the ammonium chloride solutions which have been used to treat the waste metal dust. In the Burrows method, powdered zinc dust is added to the ammonium chloride solutions and an electrochemical reaction results in which lead in elemental form deposits on the surface of the powdered zinc dust. For this reaction to proceed, a large surface area of zinc initially must be present because as the lead covers the zinc dust particle, the particle becomes no longer available for the electrochemical reaction. For this reason, very fine powder is used. However, in the Burrows method as disclosed, there is a major disadvantage in that the powdered zinc dust, when added to the solutions, immediately aggregates to form large clumps which sink to the bottom of the vessel. Rapid agitation does not prevent this from happening. Because of the aggregation of zinc, a large amount of zinc must be added to remove all of the lead, a poor practice for economic reasons. Further, if it is desired to separate the lead and some cadmium from the zinc so that all of these metals can be sold or reused, the higher the zinc concentration in the metals, the larger the mass to be processed per unit mass of zinc.
Thus, there exists a need for a method which will allow the recovery of elemental lead, cadmium, and other metals from industrial waste streams which allows the powdered zinc dust to remain dispersed in the solution so as to minimize the amount of zinc dust needed to remove lead, cadmium and other metals. Minimizing the amount of zinc required increases the economy of the process first by reducing the quantity of zinc needed, second by reducing the mass of material to be processed, and third by allowing the removal of a proportionally greater quantity of lead and cadmium.